The present invention relates to twist drills and more specifically to such twist drills for use in producing and enlarging holes in metal workpieces.
A conventional twist drill includes a pointed end that is essentially conical with surfaces leading to a chisel point at its center. The operation of a twist drill typically involves a combination of two metal removing processes. The metal to be removed directly beneath the chisel point cannot be removed totally by a cutting action in that the precise center of the drill is subjected only to axial motion. The result is that the material is deformed rather than cut, in a manner similar to deformation caused by a metal punch. As the distance increases from the exact center of the drill, so does the rotational motion of the cutting lips. The lips may then effectively operate as cutting surfaces. However, the conical surfaces of the point causes outward flow of chips toward the shoulders of the drill. This causes a binding effect and reduces efficiency of the drill as a cutting tool. The cutting lips may produce a conventional chip formation and would operate rather effectively were it not for the poor cutting capacity of the chisel point.
It therefore becomes desirable to obtain a twist drill with a point design that will maximize efficiency of the cutting lips while eliminating outward chip flow initiated at the chisel point.